Some image forming apparatuses are connectable to, as an option, a post-processing device which performs a post-process on a sheet discharged from the image forming apparatuses. An image forming apparatus provided with a post-processing device constitutes an image forming system. Examples of the post-process encompass: stapling sheets; discharging a sheet to a designated sheet output tray out of a plurality of sheet output trays; and offsetting a sheet discharging position on a sheet output tray.
Patent Document 1 (Japanese Unexamined Patent Application Publication, Tokukai, No. 2005-321482 (published on Nov. 17, 2005)) discloses such an image forming system which has the following arrangement: (i) before a post-processing device completes preparation for a change in a post-process, an image forming apparatus starts the second image forming job; and (ii) immediately after the post-processing device completes the preparation for the change in the post-process, the post-processing device starts the post-process for the second image forming job.
That is, the image forming apparatus stops carrying a sheet upon completion of the first image forming job (followed by the second image forming job). After that, at an appropriate timing before completion of the preparation for the change in the post-process, the image forming apparatus starts feeding and carrying the sheet again. Then, almost at the same point as the completion of the preparation for the change in the post-process, the image forming apparatus supplies the sheet to the post-processing device.
This reduces time between (i) start of the first image forming job and (ii) completion of a post-process for the second image forming job, compared with that of a conventional apparatus.
The arrangement in Patent Document 1 can prevent such a problem that the image forming apparatus is kept in a stand-by state throughout the time when the post-processing device is preparing for a change in a post-process. The arrangement in Patent Document 1, however, does not solve the following problem: in a case where image forming jobs are performed in succession, job efficiency is decreased because the image forming apparatus delays starting the second image forming job and causes the second image forming job to be in a stand-by state so that time required to perform a post-process for the first image forming job is secured.